Our invention relates to a low power indicating circuit, and particularly to such a circuit for use with an existing power control circuit for a radio transmitter or the like.
In radio communication services, and particularly in mobile radio telephone services, it is desirable to know if the output power of a radio transmitter has fallen below a minimum acceptable value. Such a minimum acceptable value is essential, or at least desirable, in order to provide acceptable or high quality radio communication. Such a minimum acceptable value is also essential to protect a radio transmitter whose radio frequency power output may drop because of some circuit fault, such as thermal runaway of the power amplifier device. In addition, it is desirable to provide an indication when a radio transmitter output power has fallen below a minimum acceptable value. In the case of mobile radio telephone service, such an indication can be used to alert a user that his communication is not or may not be reaching a remote location, or can be used to automatically transfer the radio communication to another radio transmitter. Upon such a transfer, the radio transmitter whose output power has fallen can be taken out of service and its fault or problem corrected.
Radio transmitters are frequently provided with an output power control circuit which maintains the output power of a radio transmitter or the like at a selected magnitude. Hence, it is desirable to utilize as much of a power control circuit as possible to provide a low power indication.